Obvious? Wow... where were you when the Apostles needed your wisdom? You see, you know these things because you have been taught through the wisdom of the Church. The Church in its beginnings didn't think this was all so obvious, though. Let's read of the discussion in its entirety...
Acts 15:1 Some men came down from Judea and began teaching the brethren, “Unless you are circumcised according to the custom of Moses, you cannot be saved.” 2 And when Paul and Barnabas had great dissension and debate with them, the brethren determined that Paul and Barnabas and some others of them should go up to Jerusalem to the apostles and elders concerning this issue. 3 Therefore, being sent on their way by the church, they were passing through both Phoenicia and Samaria, describing in detail the conversion of the Gentiles, and were bringing great joy to all the brethren. 4 When they arrived at Jerusalem, they were received by the church and the apostles and the elders, and they reported all that God had done with them. 5 But some of the sect of the Pharisees who had believed stood up, saying, “It is necessary to circumcise them and to direct them to observe the Law of Moses.”There was great dissension and discussion over the issue and there was no authority outside of their own as the Church for them to consult. Just as you have done by saying "it was obvious" but offering no Scripture to show the Will of God, they had to discern among themselves through the Spirit to "add" this doctrine for the edification of the Church. Nothing here says that the gentiles weren't expected to be saved. In fact, St Peter reminded them of his mission to the gentiles. The discussion was regarding the process the gentiles should follow... did the gentiles have to first essentially become Jews? Scripture didn't tell them. The Holy Spirit did. They promulgated a binding new doctrine for all the faithful through their authority as Apostles, not leaning on an authority of Scripture.6 The apostles and the elders came together to look into this matter. 7 After there had been much debate, Peter stood up and said to them, “Brethren, you know that in the early days God made a choice among you, that by my mouth the Gentiles would hear the word of the gospel and believe. 8 And God, who knows the heart, testified to them giving them the Holy Spirit, just as He also did to us; 9 and He made no distinction between us and them, cleansing their hearts by faith. 10 Now therefore why do you put God to the test by placing upon the neck of the disciples a yoke which neither our fathers nor we have been able to bear? 11 But we believe that we are saved through the grace of the Lord Jesus, in the same way as they also are.”
12 All the people kept silent, and they were listening to Barnabas and Paul as they were relating what signs and wonders God had done through them among the Gentiles.
As Douglas has indicated........the question was, "Should adult Gentile converts be required to undergo circumcision to become part of the New Testament Church." The faction of Pharisees (led by James) [Acts 15:5][Galatians 2:12] felt they must.....according to the Law of Moses. Well.....what did Moses have to say about that?
Moses didn't say much about circumcision but received instructions from Yahweh in two different places [Exodus 12:48] and [Leviticus 12:3]. The part that the Pharisees were having problems with was obviously the first passage:
48And when a stranger shall sojourn with thee, and will keep the passover to the LORD, let all his males be circumcised, and then let him come near and keep it; and he shall be as one that is born in the land: for no uncircumcised person shall eat thereof.
Paul and Barnabas were celebrating Passover in Antioch and were allowing uncircumcised, adult, Gentile converts to participate. If this were not the case then the Pharisees would have no special problem with Gentile converts coming into the Church. It was only this qualification that had anything to do with the "Law of Moses" [Acts 15:5]. They (The Pharisee faction) insisted that these adult, male Gentiles undergo circumcision for this.....and only this reason. It was this demand by the circumcision group that caused the entire Council to be convened [Acts 15:2]. This was the only question at the council.
Peter spoke of significant things.....how the Holy Spirit had been given to the Gentiles (without circumcision) [15:8] and how no distinction was evident [15:9]. He also related a physical condition that the Apostles, the Pharisees nor their fathers had been able to bear.....they were all circumcised as infants.
To understand just what Peter was getting at let's look at [Genesis 34:13-17].
13And the sons of Jacob answered Shechem and Hamor his father deceitfully, and said, because he had defiled Dinah their sister: 14And they said unto them, We cannot do this thing, to give our sister to one that is uncircumcised; for that were a reproach unto us: 15But in this will we consent unto you: If ye will be as we be, that every male of you be circumcised; 16Then will we give our daughters unto you, and we will take your daughters to us, and we will dwell with you, and we will become one people. 17But if ye will not hearken unto us, to be circumcised; then will we take our daughter, and we will be gone.
All of the men of the city consented to be circumcised and Simeon and Levi (brothers of Dinah) killed them all [34:25] while they were recovering....still in agony....and unable to defend themselves. This is what Peter was referring to......"what we nor our fathers had ever been able to bear".....and we should not ask it of adult, male Gentiles who wish to join with us in our worship.
The discussion was regarding the process the gentiles should follow... did the gentiles have to first essentially become Jews? Scripture didn't tell them. The Holy Spirit did. They promulgated a binding new doctrine for all the faithful through their authority as Apostles, not leaning on an authority of Scripture.
I think they did rely on scripture.....and then used common sense. I also believe that new born Gentile babies were routinely circumcised for health reasons. I'm sure their parents were encouraged by the Apostles to maintain this custom. Adult, male converts continued to be excused for obvious reasons.